Radiant energy mark sensor

ABSTRACT

The radiant energy mark sensor herein makes use of Brewster&#39;&#39;s Law and adds a polarizing screen in the light path to improve the radiant energy contrast between marks and a background. The incident beam of light strikes the background at or near Brewster&#39;&#39;s angle. A polarizing screen is placed in the path of the reflected light. Light reflected from the background will be polarized, and the polarizing screen can null out that light. Light reflected from marks will not be polarized, and some of the light will be passed by the polarizing screen. In an alternative configuration, the polarizing screen is placed in the incident light path to control the amount of light reflected from the background and from the mark.

United Stat Holdaway RADIANT ENERGY MARK SENSOR Inventor: Daniel 0.Holdaway, Longmont,

Colo.

Assignee: International Business Machines Corporation, Armonk, N.Y.

Filed: Dec. 28, 1970 Appl. No.: 101,704

References Cited UNITED STATES PATENTS Stites ..350/152 X Smith ..340/174.1 M

[5 7] ABSTRACT The radiant energy mark sensor herein makes use ofBrewsters Law and adds a polarizing screen in the light path to improvethe radiant energy contrast between marks and a background. The incidentbeam of light strikes the background at or near Brewsters angle. Apolarizing screen is placed in the path of the reflected light. Lightreflected from the background will be polarized, and the polarizingscreen can null out that light. Light reflected from marks will not bepolarized, and some of the light will be passed by the polarizingscreen. In an alternative configuration, the polarizing screen is placedin the incident light path to control the amount of light reflected fromthe background and from the mark.

9 Claims, 4 Drawing Figures 1 RADIANT ENERGY MARK sENson BACKGROUND OFTHE INVENTION This invention relates to radiant energy mark sensing.More particularly, the invention relates to mark sensing where thebackground surface carrying the marks is a transparent material.Transparent material is defined herein as a material having an index ofrefraction for the wavelength of radiant energy being used.

In the past, marks coated on transparent backgrounds have been sensed bydirecting white light onto the surface and monitoring the reflectedlight with a photodetector. The sensing of marks is dependent upon thedifferences in reflectivity of the mark and the transparent background.The optimum contrast (change in light intensity detected atphotodetector) obtainable in this conventional sensor has been in theorder of to 1.

In a tightly controlled environment, a contrast ratio of 10 to l isquite acceptable. However, in an environment where components are massproduced, a 10 to l contrast ratio may not be sufficient. For example,to detect the presence of a mark, the output of the photodetector wouldbe monitored with a threshold detecting circuit. When the contrast ratiois 10 to l, the threshold would be set such that approximately a 6 or 7to 1 ratio in reflected light intensity difference would exceed thethreshold. When the components of various systems utilizing this type ofsensor are not tightly matched, then it is possible for one light sourceto be many times brighter than another light source. If the samethreshold in the detecting circuit were used, reflected light producedby a bright light source might always be greater than the thresholdintensity, irrespective of whether a mark or background was beingsensed. Therefore, the manufacturer is forced to use expensivecomponents and also an extensive matching and calibrating operation toinsure that thresholds are properly set to detect the presence of markson the surface being scanned.

It is an object of this invention to increase the contrast ratio betweenbackground and marks by an order of magnitude.

It is another object of this invention to sense marks coated ontransparent backgrounds.

SUMMARY OF THE INVENTION In accordance with this invention, the aboveobjects are accomplished by directing the beam of incident light ontothe background at or near Brewsters angle. In addition, a polarizingscreen is placed in the light path. The polarizing screen is adjusted soas to inhibit light reflected from the background or to inhibit lightfrom reflecting from the background.

As another feature of the invention, light refracted into a transparentbackground is absorbed or scattered by a non-reflective coating on theback of the transparent material.

As another feature of the invention, the polarizing screen, placed inthe reflected light path, is also a wavelength filter. The filteringoperation is useful in preventing the passage of ambient light to thephotopickup device in the reflected light path. In other words, thefilter is selected to pass only light of the wave length of the incidentlight. Other light which may also be illuminating the surface beingscanned is filtered out.

The great advantage of this invention is that the contrast ratio betweenthe reflective background and the mark has been increased to 40 to 1. Asa result of this very high contrast ratio, the components used to sensethe marks do not need to be closely matched. The components can be lowin cost, and no previous testing or calibration of components isnecessary. With a 40 to l contrast ratio, the threshold used by thephotodetecting circuit can be chosen so that all components even withoutcalibration will produce intensity variations that will enable thephoto-detecting circuit to distinguish between marks and background.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS FIGS. 1A and 1B show one preferredembodiment of the invention indicating the light paths when the beam oflight strikes the transparent background and when the beam of lightstrikes a mark attached to the background.

DESCRIPTION FIGS. 1A and 1B show the preferred embodiment of theinvention wherein the polarizing screen is placed in the reflectivelight path. The object being scanned by the light beam is made up oftransparent substrate, or background material, 10. This material couldbe a polyester, glass, or any other material having an index ofrefraction for the wave length of radiant energy being used. Attached tothis substrate 10 are the reflective marks 12. The marks are formed bycoating a highly reflective metal on the surface of the substrate 10.Some examples might be aluminum or silver. The back of the substrate 10is coated with a light absorbent material 14. This material could beiron oxide or simply a flat black paint.

The incident light 16 is directed onto the substrate 10 in FIG. 1A. Aportion of the light is refracted into substrate 10 and absorbed by thelight absorbing layer 14. The remaining light is reflected from thesubstrate 10 to the polarizing screen 18. A photodiode orphototransistor 20 is mounted behind the polarizing screen 18 to detectthe presence of any reflected light -22 passed by the screen 18.

As shown in FIG. 1A, the incident light 16 strikes the substrate 10, andthe reflected light is blocked by the polarizing screen 18 so that itdoes not reach the photopickup 20. This is accomplished by having theincident beam of light 16 strike the substrate 10 at or near Brewstersangle and placing the polarizing screen 18 to null out the polarizedreflected light. As is well known, Brewsters angle is given by thefollowing expression: n /n tan 6 where n is the index of refraction ofthe substrate 10, n 'is the index of refraction of the incident medium(usually air where n 1), and is the angle of incidence of the light beam16. While the above expression gives the angle of incidence for optimumpolarization of reflected light, the exact angle is not critical.Successful operation can be achieved if the angle of incidence is withina range specified by Brewsters angle 20 and Brewsters angle +5". Thereflected light 22 is plane polarized perpendicular to the plane ofincidence. The polarizing screen 18 is set to pass light only in theplane of incidence, and, therefore, little or reflected light is passedby the polarizing screen 18 in FIG. 1A.

' The refracted light 24 in FIG. 1A passes through the substrate and isabsorbed in the light absorbent material 14 or scattered at theinterface between substrate 10 and material 14. If the light were notscattered at this interface, it would reflect back out the top of thesubstrate 10, through the polarizing screen 18 and tothephotodetector20. The refracted light 24, of course, would have a component in theplane of polarization of polarizer 18 and, thus, would pass throughpolarizing screen. Therefore, the light absorbing or scattering material14 is provided to insure that negligible reflected light from inside thesubstrate 10 will reach the photodetector 20.

In FIG. 1B, the incident light 16 strikes a reflective mark 12.Brewsters Law is no longer satisfied. The reflected light 26 from themark 12 is not polarized. Accordingly, the polarizing screen 18 willpass some component of the reflected light 26. Light passed by thepolarizing screen 18 is picked up by the photodetector 20. The output ofthe photo-detector is monitored by electronic circuitry not shown, and,when the electrical signal exceeds a preset threshold, the mark has beendetected. I

In FIG. 2, the invention is applied to the detection of aluminum stripsattached by adhesive to the back of magnetic tape. The substrate 10 is aflexible web, such as polyester, the reflective mark 12 is a strip ofaluminum and the light absorbing layer 14 is usually some composition ofmagnetic particles in a binder. The light source 28 may be a smallincandescent bulb. The source 28 is placed so that light from the bulbwill strike the substrate 10 at or near Brewsters angle. It has beenfound that the optimum angle of incidence for a polyester is 59;although, any angle between 40 and 65 will polarize the lightsufficiently for use in this invention. Light reflected from thesubstrate 10 or the mark 12 must pass through the polarizing screen 30in FIG. 2 before it can reach the phototransistor 32.

The polarizing screen 30 is also made up of a material that passes onlyinfrared radiation. Thus, the screen 30 also acts as an optical filtertuned to light from the source 28. This prevents any ambientillumination of substrate 10 or mark 12 from reaching thephototransistor 32. Of course, the source 28 must have a strongcomponent in the infrared portion of the spectrum.

The operation of the mark sensor for magnetic tape in FIG. 2 isidentical to the operation previously described with reference to FIGS.1A and 1B.

Yet another application of the invention is shown in FIG. 3 where themark sensor is used to detect marks on an optical tachometer. Thetransparent substrate 10 takes the shape of a wheel mounted on a shaft34. The back of the substrate 10 is again coated with a light absorbentor scattering material 14, and the front of the substrate 10 hasreflective marks 12 attached. A light source 36 and a photocell 38 aremounted in their respective holders 40 and 42. In addition, a polarizingscreen 44 is placed in front of the light source 36 in the holder 40.

In operation, light from the source 36 is polarized by polarizer 44 andis then incident at Brewsters angle on the substrate 10. The plane ofpolarization of the incident light 46 is such that at Brewsters angleall of the light will be refracted into the transparent material 10 andabsorbed or scattered by the light absorbing material 14. In otherwords, the angle of incidence 0 of the light 46 is again given by theexpression:

where n is the refractive index of the substrate 10 and n is the indexof refraction of the incident medium. Also, the incident light 46 mustbe linearly polarized perpendicular to the plane of incidence so thatsubstantially all the incident light is refracted into the substrate 10.

In this mode of operation, incident light will only be reflected to thephotocell 38 when the light strikes a reflective mark 12. When the lightstrikes the substrate 10, it is refracted and absorbed or scattered.

It will be appreciated by one skilled in the art that there are manyapplications other than the two described in FIGS. 2 and 3 for thisinvention. Also, a great variety of light sources, polarizing screens,and photopickup devices could be utilized. In addition, any transparentsubstrate along with reflective marks could be used. The addition of thelight absorbent or scattering material 14 is desirable, but is notcritical to operation. The function of preventing light from beinginternally reflected in the substrate 10 and back to the photopickupcould be accomplished by several expedients, such as making the backsurface of the substrate l0 irregular. Thus, while the invention hasbeen particularly shown and described with reference to preferredembodiments thereof, it will be understood that various changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. Radiant energy mark sensing apparatus for detecting the presence ofreflective marks on a substrate transparent to the radiant energycomprising:

means for directing radiant energy onto the substrate and the reflectivemarks at an angle of incidence such that radiant energy reflected fromthe substrate is linearly polarized perpendicular to the plane ofincidence and radiant energy refracted into the substrate is linearlypolarized in the plane of incidence, and radiant energy reflected fromsaid marks is substantially unchanged in polarization and contains atleast radiant energy components oriented in the plane of incidence;

means for detecting radiant energy reflected from the substrate and fromthe reflective marks;

means for inhibiting reflected radiant energy linearly polarizedperpendicular to the plane of incidence 6 from passing from thesubstrate to said detecting same radiant energy, comprising the stepsof: means while passing to said detecting means radiilluminating thesubstrate and the reflective areas ant energy components oriented in theplane of inwith the radiant energy at an angle of incidence cidencereflected from the reflective marks so that Such that the radiant nergye e d y he ubthe contrast ratio between detected radiant energy 5 strateis i y Polarized While radiant energy from marks and detected radiantenergy from the reflected y the reflective area is unPOlaTiZed;substrate is enhan d, analyzing the polarization of the reflectedradiant 2. The apparatus of claim 1 wherein said inhibiting energywhereby y Polanzed radlam energy means comprises; reflected by thesubstrate is distinguished from unmeans for linearly polarizing in theplane f i0 polarized radiant energy reflected by the reflective cidencethe incident radiant energy supplied by marks said directing means sothat the linearly polarized The method 6 i addltlon the Step incidentradiant energy Striking the Substrate at absorbing or scattering radiantenergy refracted into said angle will be substantially refracted intothe the substrate byth? Substrate. Substrate 8. The method of claim 6wherein said analyzing step 3. Th 21 ratu of cl h ii'b' means z zi S am1 w erem sald m 1 filtering the radiant energy reflected by thesubstrate p and by the reflective areas with a polarizing filter meansfor analyzing the polarization of reflected oriented to intercept thelinearly polarized radiant radiant energy so that non-linearly polarizedenergy reflected by the substrate;

detecting the filtered reflected radiant energy whereby the presence ofreflective areas is indicated by a substantial increase in the radiantenergy detected. 25 9. A method for distinguishing areas reflective toradiant energy from a support substrate transparent to reflected radiantenergy from reflective marks is passed to said detecting means whilelinearly polarized radiant energy reflected from the substrate isblocked from reaching said detecting means.

4. The apparatus of claim 1 and in addition: means for filtering thereflected radiant energy so the same radiant energy comprising the stapsof: that only radiant energy with a wave-length within illuminating thesu strate and the re ective areas the spectrum of the radiant energysupplied by said with linearly polarized radiant energy where thedirecting means may be passed to said detecting ngle of incidence of theradiant energy on the means. substrate and reflective areas is such thatlinearly 5. The apparatus of claim 1 and in additi polarized radiantenergy incident upon the submeans for absorbing a d att i r di t energystrate is refracted into the substrate while the radirefracted into thesubstrate so that radiant energy am energy lncldem p the feflectlve area15 refracted into the substrate cannot reflect back to Yeflcted iddetecting means detecting the reflected radiant energy whereby the 6 Amethod for distinguishing areas reflective to presenc? f the reflective1S lndlcated y a radiant energy f transparent Substrates When thesubstantial increase in the radiant energy detected. areas are mountedon substrates transparent to the

1. Radiant energy mark sensing apparatus for detecting the presence ofreflective marks on a substrate transparent to the radiant energycomprising: means for directing radiant energy onto the substrate andthe reflective marks at an angle of incidence such that radiant energyreflected from the substrate is linearly polarized perpendicular to theplane of incidence and radiant energy refracted into the substrate islinearly polarized in the plane of incidence, and radiant energyreflected from said marks is substantially unchanged in polarization andcontains at least radiant energy components oriented in the plane ofincidence; means for detecting radiant energy reflected from thesubstrate and from the reflective marks; means for inhibiting reflectedradiant energy linearly polarized perpendicular to the plane ofincidence from passing from the substrate to said detecting means whilepassing to said detecting means radiant energy components oriented inthe plane of incidence reflected from the reflective marks so that thecontrast ratio between detected radiant energy from marks and detectedradiant energy from the substrate is enhanced.
 2. The apparatus of claim1 wherein said inhibiting means comprises: means for linearly polarizingin the plane of incidence the incident radiant energy supplied by saiddirecting means so that the linearly polarized incident radiant energystriking the substrate at said angle will be substantially refractedinto the substrate.
 3. The apparatus of claim 1 wherein said inhibitingmeans comprises: means for analyzing the polarization of reflectedradiant energy so that non-linearly polarized reflected radiant energyfrom reflective marks is passed to said detecting means while linearlypolarized radiant energy reflected from the substrate is blocked fromreaching said detecting means.
 4. The apparatus of claim 1 and inaddition: means for filtering the reflected radiant energy so that onlyradiant energy with a wave-length within the spectrum of the radiantenergy supplied by said directing means may be passed to said detectingmeans.
 5. The apparatus of claim 1 and in addition: means for absorbingand scattering radiant energy refracted into the substrate so thatradiant energy refracted into the substrate cannot reflect back to saiddetecting means.
 6. A method for distinguishing areas reflective toradiant energy from transpareNt substrates when the areas are mounted onsubstrates transparent to the same radiant energy, comprising the stepsof: illuminating the substrate and the reflective areas with the radiantenergy at an angle of incidence such that the radiant energy reflectedby the substrate is linearly polarized while radiant energy reflected bythe reflective area is unpolarized; analyzing the polarization of thereflected radiant energy whereby linearly polarized radiant energyreflected by the substrate is distinguished from unpolarized radiantenergy reflected by the reflective marks.
 7. The method of claim 6 andin addition the step of: absorbing or scattering radiant energyrefracted into the substrate by the substrate.
 8. The method of claim 6wherein said analyzing step comprises the steps of: filtering theradiant energy reflected by the substrate and by the reflective areaswith a polarizing filter oriented to intercept the linearly polarizedradiant energy reflected by the substrate; detecting the filteredreflected radiant energy whereby the presence of reflective areas isindicated by a substantial increase in the radiant energy detected.
 9. Amethod for distinguishing areas reflective to radiant energy from asupport substrate transparent to the same radiant energy comprising thesteps of: illuminating the substrate and the reflective areas withlinearly polarized radiant energy where the angle of incidence of theradiant energy on the substrate and reflective areas is such thatlinearly polarized radiant energy incident upon the substrate isrefracted into the substrate while the radiant energy incident upon thereflective area is reflected; detecting the reflected radiant energywhereby the presence of the reflective areas is indicated by asubstantial increase in the radiant energy detected.